After an injury or disease of the central nervous system (CNS) parts of the body will be functioning normally, but parts of the body will be paralyzed. Many muscles will be connected to the CNS below the level of injury; thus, they are innervated, but functionally paralyzed (not controllable volitionally). Many sensory pathways are connected to the CNS, yet their function is lost or modified because the information that they carry is not relayed to the corresponding higher centers within CNS.
Functional electrical stimulation (FES) can be considered as a bypass of the impaired sensory-motor mechanisms. FES must provide synergistic actions of many muscles, full control over the each of the muscles by following the findings about the size principle, recruitment order, and recruitment rate, and it should also include sensors feedback for both operation of the system and cognitive awareness of the action if it is to be effective. In parallel, it must be practical to allow independent and effective daily use by a person with disability.
In biological systems the regulation of the strength of a motor response is done through the number of active motor nerve fibers and the rate at which they trigger action potentials: recruitment and temporal summation, respectively. In a physiological contraction, the recruitment order is fixed; slow, fatigue-resistant motor units are active at a lower voluntary effort than larger, fast, fatigable units. The second mechanism affecting the overall force developed by the muscle is temporal summation. The frequency at which the generated muscle forces are sufficiently smooth is known as fusion frequency. The point at which fusion is achieved depends upon the speed of contraction of the activated muscle fibers, and therefore ultimately upon the level of recruitment. In biologically innervated muscles the motor neurons act asynchronously at frequencies that are typically bellow 5 pulses per second; yet, the net effect is a smooth contraction.
In paralyzed muscles electrical stimulation is delivered to innervation pathways to replace the missing biological control signals in bursts of pulses. In an externally induced recruitment, the recruitment order is not known a priori, but depends upon the variables of position and geometry as well as fiber size. An inverse order of electrically induced recruitment is typical when applying FES; the largest fibers are being easily excited, compared with small fibers. This implies that the recruitment has to be considered at all times in order to provide controlled and graded externally induced activation. The recruitment of nerve fibers with increasing stimulus pulse amplitude or duration is nonlinear. For this reason, a linear increase of muscle output force cannot be achieved by a linear change in the input. In externally activated muscles it is impossible with present technology to mimic normal activation, since it is rather difficult to individually activate motor units; hence, the fusion occurs at about 20 pulses per second. Increasing the stimulus frequency above the fusion frequency to the level of tetanus results in a further increase in force. Up to 40 or 50 percent of the maximum muscle force may be regulated by temporal summation from fusion to tetanus.
The force generated by the muscle is directly related to the intensity of stimulation. The intensity of stimulation is directly related to the amount of charge delivered by a pulse. The minimum level of charge is determined by chronaxia, or I-T (amplitude of pulse I vs. pulse duration (width) T). Thereby, the amplitude modulation (AM) or pulse width modulation (PWM) governs the level of recruitment, that is, the force. The recruitment modulation should guaranty reproducibility; therefore consider changes that are likely to occur during prolonged periods of activation. Most, if not all, FES systems activate simultaneously many motor units.
The sensory system in humans operates as an extra large neural network that has been trained through numerous trials and errors. The biological sensory system components provide frequency coded series of binary information, and the process of fusion of this information is not completely understood and described in literature. The inputs that play major role are vision, vestibular system, auditory system, and somatosensory system (exteroception and propriception). Natural control operates in space that is qualitatively described (e.g., hand in contact with an object, elbow fully extended, body erected, etc.). In contrast, artificial sensors systems transform a physical quantity into an useful electrical signal that carries quantitative information about the physical quantities in question. In a highly reduced version of an artificial sensor, the single threshold method applied to the output is a binary signal; hence, if frequency coded it would be a replica of a sensory cell. Somatosensory systems of a human communicate with the brain via the spinal cord, and visual, auditory, and vestibular systems directly. The spinal cord serves both as a relay and as an integration and processing mechanism during the translation of the signal from the periphery to the brain.
In summary, the task for generating functional movement is extremely complex: replacement of a controller that acts on a multi-actuator system based on a multi-sensor system and heuristically optimized rules. From the engineering point of view the system to be controlled is multi input, multi output, time varying highly nonlinear system in which individual parameters can only be estimated based on non-perfect models.
Following elements are known from the prior art (Special issue J Automatic Control, Vol 18(2), 2008):                Models of electrical field (current density) distribution when electrical stimulation is applied;        Surface electrodes with adhesive hydrogel in various sizes;        Textile substrate single pad or multi-pad electrodes in various sizes with hydrogel or made conductive when wet;        Various garment type alignments for the electrodes;        Microprocessor based current regulated or voltage regulated multi-channel stimulators suitable for safe application of surface electrical stimulation;        Electronic stimulators with up to eight channels with numerous predefined stimulation patterns for exercise and limited number of functional movements (grasping, walking);        MEMS and EMG sensors that measure acceleration, position, joint angle, pressure, force, and level of muscle effort;        Model-based controllers that drive the multi-channel stimulator to allow the tracking of predefined trajectories;        Finite-state controllers that operate based on feedback and predefined synergies.        
However, there are no practical systems which can be used for therapies of individuals with hemiplegia, multiple sclerosis, cerebral paralysis, incomplete paraplegia, incomplete tetraplegia, tremor, and other movement disorders. The reasons for the lack of these systems are: non sufficiently selective stimulation, fast occurring muscle fatigue, not sufficiently adaptive and robust control algorithms to allow instrumental adaptation to the needs of patients, problems with donning and doffing of the system, and not sufficiently functional in operation.